One mistake I made was that I had created an Instrument track, not an Audio track. So I deleted it and made an Audio track. The "not a ProTools Session" error message no longer appears. Instead, the window shown in the attachment appears where nothing works. Clicking on Add Files, or Add Clip only un-highlights the desired mp3 choice. Done only shuts the window. I've tried dragging the mp3 file onto the track, as I've always done before in other sessions, but nothing happens, nothing at all.